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Image formation - Thin lenses

Image formation by thin lenses

1) Effects of a lens on a beam of light parallel to the principal axis

A beams of light whose rays are parallel to the principal axis propagates toward a thin diverging lens.
After the lens, the beam of light widens and rays move away from the principal axis: the beam of light is diverging.
If the diverging lens is replaced by a converging lens then the beam of light gets less wide and rays get closer to the principal axis: the beam of light is converging.
For simplification:
The beam of light before the lens is called " incident beam "
- The beam of light after the lens is called " emergent beam "


2) Characteristics of a converging lens

The point where converge the rays of an incident beam parallel to the principal axis is denoted by F and called focal point.
Distance between the optical center and the focal point is denoted by f and called optical lenght.

The focal length can easily be determinate using a parallel incident beam. A screen, after the lens, must be moved until the spot of light gets the tiniest: the screen is then placed at the focal point.

3) Formation of an image with a converging lens

To form an image of a bright object with a converging lens, a screen must be placed behind the lens and its distance has to be adjusted to obtain a clear image.
The formation of an image is possible only if the distance between the screen and the object is greater than the focal length.
The image formed is then inverted, and placed after the focal point of the lens.